also add preliminary script to generate dependency list.
we should use this for compiling in the future.

NSVEparticles test works.

by default a new field object (pressure) is generated every time the
acceleraation is computed in vorticity_equation. I now allow for an
externally allocated pressure field to be used, thus avoiding the
creation of a new field object every niter_part.

basic stats can now be read even in the absence of the main simname.h5
file, using just the cache file.

I'm not sure how much sense this makes, but I am now explicitly dividing
the computed spectrum by dk, such that the integral of E(k)dk is the
total energy.

I know the arithmetic mean thing is more polite, but I'm going to use
the old version, so that the test still passes as is.
And everyone does it this way anyway...